1. Field of the Invention
This invention relates to an optical head for recording information in, reproducing the information recorded in or erasing information from information tracks formed on a recording medium.
2. Description of the Related Art
Typically, a conventional optical head for use in an optical disk drive unit includes a focus drive means for moving an objective lens in the direction parallel to an optical axis thereof (hereunder sometimes referred to as the focusing direction) in order to follow axial deflection of a disk, a tracking drive means for moving the objective lens in the radial direction of the disc (hereunder somtimes referred to as the tracking direction) to follow an information track having a dynamic radial runout therein and an access drive means for moving an optical unit containing the focus drive means and the tracking drive means to an information track to be followed (hereunder sometimes referred to simply as a target track). The objective lens is supported by the optical unit through an elastic supporting member such as a plate spring. Further, each of the focus drive means, the tracking drive means and the access drive means is generally constructed in such a manner to arrange a movable coil in magnetic flux of a magnetic circuit and generate driving force by energizing the coil.
Further, a seek operation of an optical head following a track is performed as follows. First, the tracking drive means is once disconnected or deenergized while the focus drive means operates to effect a focus control operation. Then, the access drive means is energized to move the optical head to the neighborhood of the target track. Thereafter, the tracking drive means is energized to be in operation. Thus, is read the address of the track which the optical head reaches. If the read address is not equal to that of the target track, what is called a "track jumping operation" is performed by using the tracking drive means to find the target track. Finally, the optical unit reaches the target track and thus the seek operation is completed.
When the optical head is moved by using the access drive means, it is preferable that the objective lens does not rock in the tracking direction but moves together with the optical unit, i.e., the objective lens and the optical unit are rigidly connected with each other. This is because of the fact that when the objective lens rocks in the tracking direction, there occur disadvantages that the address of the track, which the optical unit reaches, is largely different from that of the target track, that what is called a "tracking-pull-in operation" becomes unstable and that a seek time increases.
Further, it is desirable that the objective lens is supported in the tracking direction in the vibration state of a system of single degree of freedom. If supported in the vibration of a system of multi-degree of freedom, there occur many resonance modes and thus a tracking control system becomes unstable.
As an example of a conventional system for suppressing the rocking motion of the objective lens in the tracking direction at the time of moving the optical unit, is cited a system disclosed in Japanese Patent Provisional Publication No. 63-298821 Official Gazette. In this system, the whole of an optical system having both an objective lens and a tracking drive means is included in an optical unit provided with an access drive means. Further, a tracking control means for controlling the movement of the objective in the tracking direction is mounted on the optical unit. This tracking control means is made up of a solenoid type electromagnet. When the electromagnetic is energized, a solenoid pin is protruded and is engaged with a groove of a supporting member for supporting the objective lens in such a manner to be able to move in the tracking direction, thereby controlling the movement of the objective lens. That is, this system energizes the tracking control means to suppress the rocking motion of the objective lens at the time of performing an access operation. In contrast, during the tracking operation, this system deenergizes the tracking control means and effects a tracking control operation. This conventional system, however, has defects that if the mass of the whole optical unit, which moves at the time of effecting an access operation as above described, is not far greater than that of the objective lens, the objective lens is not supported by a system of single degree of freedom and further the optical unit a becomes a system of double degree of freedom and as a result the tracking control operation becomes unstable, that it is therefore difficult to decrease the mass of such a movable portion at the time of getting an access to an information track and that in addition, the access drive means is large in size and requires considerable drive electric power. That is, the prior art has drawbacks in that it is difficult to miniaturize an optical disk drive unit and decrease power consumption thereof.
Regarding the miniaturization of an optical disk drive unit, a separate type optical head has been proposed, in which the optical system is separated from the residual portions thereof and only the focus drive means, the tracking drive means and a part of optical components such as a reflecting mirror are included in a movable unit provided with the access drive means, thereby decreasing the weight of an optical disk drive unit. However, in this separate type optical head, there arises a difficulty that as the weight of the movable unit becomes smaller, the objective lens becomes supported by a system of double degree of freedom and the tracking control operation becomes unstable. A countermeasure for solving this problem of the unstableness in the tracking control operation, has been proposed in Japanese Patent Provisional Publication No. 2-68730 Official Gazette. It is another separate type optical head which comprises a movable bobbin having an objective lens and a tracking drive means, a transfer guide means, and a carriage having mass almost equal to the mass of the movable bobbin and connected to the movable bobbin through a plate spring and provided with a reflecting mirror and an access drive means guided in the transferable manner by the transfer guide means. In this optical head, the access drive means and the tracking drive means are simultaneously operated during the tracking control operation to electrically correct the movement of the system of a double degree of freedom supporting the objective lens. This conventional optical head, however, is disadvantageous in that the tracking control operation becomes complex and that the state of the tracking control operation changes due to a slight deviation in the spring constant of the plate spring or to variation in a sliding resistance of the transfer quide means to guide the carriage with the result that the state of the tracking control operation may become unstable. Therefore, the problem of the unstableness in the tracking control operation cannot satisfactorily be solved by this conventional optical head. Furthermore, the problem of decreasing the power consumption cannot be solved because the tracking drive means and the access drive means are simultaneously operated in this conventional optical head as above described.